A rupture of the supply line, or an uncoupling of the supply line glad hands, will result in a drop of pressure in the supply line between the trailer supply valve (24) and the relay emergency valve (25). This will trigger the emergency action of the relay emergency valve, placing the brakes into emergency application. As in the previous examples, the trailer supply valve (24) will shift to the closed position.

The operation of the tractor brakes will not be affected if the tractor protection system is in working condition.

The relay emergency valve (25) must be the no bleed-back type, so no air is lost from the trailer.